Usually, luatex get updated regularly along with ConTeXt updating. If for some reason, you want to use the latest luatex binary, you need to compile it from source code yourself. If you have difficultly compile binary from source, you can ask others to compile for you, or just wait for the binary to eventually come into the ConTeXt repo.luatex has not been updated after executingfirst-setup.sh --keep, what should I do for updating Luatex?